1. Field of the Invention
This invention relates to video signal processing circuitry for processing a composite color video signal, and is more particularly directed to a comb filter circuit suitable for processing a color video signal reproduced by means of a video tape recorder/player or VTR.
2. Description of the Prior Art
Comb filters are commonly used for processing the luminance and chrominance components of a composite color video signal reproduced from magnetic tape or other recording medium in order to eliminate undesired signals, such as so-called crosstalk. Because of its serrated characteristic, a comb filter also aids in separation of frequency-interleaved signals, and thus can be used to eliminate remnants of chrominance components from the reproduced luminance component and to eliminate remnants of the luminance component from the reproduced chrominance component.
Generally, there are two types of comb filter circuits commonly used in a VTR: a chrominance, or C-type comb filter for processing the reproduced chrominance component; and a luminance, or Y-type comb filter for processing the reproduced luminance component.
A conventional C-type comb filter is an open-loop circuit formed of a delay line with a one-horizontal-line-period delay time and a subtractive combining circuit for combining the delayed and undelayed chrominance component. Because the phase of the chrominance subcarrier signal alternates from one line to the next, the delayed and undelayed chrominance components combine constructively in the subtractive combining circuit.
The comb filter, being an open loop circuit, has a wide pass band characteristic in the vicinity of the frequency of the chrominance subcarrier. Consequently, the blocking characteristic for the luminance component is rather narrow, and undesired remnants of the luminance component are permitted to pass therethrough. For this reason the signal to noise ratio (SNR) for the chrominance component remains rather low even after processing the same through the C-type comb filter.
To combat this problem it has been previously proposed to incorporate into a C-type comb filter a feedback circuit to improve the SNR. This can comprise an additive combining circuit for combining the delayed and undelayed chrominance components, and an attenuator for feeding back the combined resulting signal from the additive combining circuit to a point in advance of the comb filter. Then a subtractive combining filter combines the attenuated feedback signal with the input chrominance signal to reduce or eliminate the undesired remnants of the luminance signal from the processed chrominance signal.
This previously proposed circuit works well only so long as there is strong vertical correlation in the luminance component of the video signal, that is, only when video information in one horizontal line interval has substantially the same value as corresponding video information in an adjacent horizontal line interval. Accordingly, the previously-proposed circuit also includes a correlation detecting circuit for detecting whether there is good vertical correlation in the luminance component and a switch connected to the attenuator for cutting the feedback signal in or out in response to the output of the detecting circuit.
However, because there is no feedback signal provided at times when a lack of vertical correlation is detected, the comb filter still has a rather low SNR at such times.
The circuit for detecting vertical correlation generally includes a delay line having a one-line-interval delay time, a subtractive combining circuit provided with the luminance component and a one-line-interval delayed version thereof, and a comparator for comprising a difference output from the subtractive combining circuit with a threshold reference level. When the subtractive output is below the reference level, high vertical correlation is indicated, and a signal is provided to cut the feedback circuit in. When the subtraction output is higher than the reference level, low vertical correlation is indicated, and the feedback circuit is cut out.
Unfortunately, a conventional delay line is used in the above circuit for detecting vertical correlation. Such delay lines have a good pass band only for frequencies in the vicinity of the chrominance subcarrier frequency. Consequently, the delayed and undelayed versions of the luminance signal that are applied to the subtractive combining circuit have substantially different characteristics. As a result, the signal provided by the correlation detecting circuit will not accurately reflect true vertical correlation in the luminance component.
A conventional Y-type comb filter is also an open-loop circuit and is generally formed of a delay line with a one-horizontal-line-period delay time and an additive combining circuit for combining the delayed and undelayed luminance component. As mentioned previously, the delay line has a band pass characteristic centered at the frequency of the chrominance subcarrier. Consequently, the the Y-type comb filter has a higher gain in the vicinity of the chrominance subcarrier than at other frequencies in the band of the luminance component. This can cause distortion and ringing in the video signal.
It has been proposed to provide a low pass filter following the Y-type comb filter. However, such a filter cancels out many desired high-frequency portions of the luminance component. Thus, conventional techniques have failed to provide a Y-type filter having a flat output characteristic.